It is well known that refractive index-gratings can be formed in germano-silicate fiber by exposure of the fiber to radiation of the appropriate wavelength. See for instance K. O. Hill et al., Applied Physics Letters, Vol. 32, page 647 (1978). See also G. Meltz et al., Optics Letters, Vol. 14, page 823 (1989), wherein the formation of Bragg gratings in germano-silicate optical fibers by a transverse holographic method is reported. J. Stone, Journal of Applied Physics, Vol. 62(11), pp. 4371-4374 (1987) reports observation of photorefractivity in GeO.sub.2 -doped silica fibers, and concludes that the effect increases with increasing GeO.sub.2 content. The above publications are incorporated herein by reference.
G. Meltz et al. (op. cit.) report forming Bragg gratings in several different fibers, with core diameters of 2.2 to 2.6 .mu.m and numerical apertures (NA) of 0.17 to 0.24, corresponding to GeO.sub.2 doping of 5 to 12.5 mol %. They also report achieving fractional index perturbations (.DELTA.n) of approximately 3.times.10.sup.-5, and Bragg filter reflectivities of 50 to 55%. D. P. Hand et al., (Optics Letters, Vol. 15(2), pp. 102-104) report achieving .DELTA.n of 7.27.times.10.sup.-5 at 488 nm after 8.5 hours exposure to 60 mW/.mu.m.sup.2 488 nm radiation. United Technologies product literature announces that the company can produce Bragg gratings in most commercially available fibers, with reflectivities in the range 10-80% (greater than 90% for custom gratings). It is generally believed that a defect is associated with the photorefractive effect in germanosilicate fiber.
Some defects in germano-silicate glass are associated with the presence of Ge.sup.2+ in the glass. In particular, it has been observed that the concentration of Ge.sup.2+ -associated defects can be decreased if sintering of a VAD-produced porous 90 SiO.sub.2 : 10GeO.sub.2 preform body is carried out in a O.sub.2 /He mixture, as compared to conventional sintering in He. It has also been observed that sintering of such a body in a H.sub.2 /He atmosphere results in a preform rod with axially varying Ge-distribution and in the presence of a luminescence center whose key components are a reduced species of Ge and H. See A. Kashiwazaki, et al., Materials Research Society Symposium Proceedings, Vol. 88, page 217 (1987).
In general, optical fiber preforms are generally produced under conditions that tend to minimize the likelihood of formation of Ge.sup.2+ -associated defects. For instance, in the MCVD process it is customary to collapse the preform such that the interior surface of the preform tube is in contact with a relatively high concentration of oxygen and small amounts of chlorine.
The relatively small photorefractive effect in typical prior art fibers necessitates relatively long exposure times and frequently results in relatively inefficient and long Bragg reflectors in the fibers. However, it would be highly desirable to be able to efficiently produce relatively short high reflectivity in-line Bragg gratings and other optical components in otherwise conventional germano-silicate optical fiber through the use of the photorefractive effect. It thus would be desirable to have available optical fiber that has a higher concentration of the relevant defect than is typically found in the analogous prior art fiber. This application discloses such fiber, and also discloses methods of producing the fiber.